(1) The Field of the Invention
The present invention relates to fibres of the modacrylic type which have a reduced inflammability but maintain a high degree of gloss even after the hydrothermal treatments such as boiling in water, dyeing, steaming. Further objects of the present invention are compositions and a process for making said modacrylic fibres by wet spinning solutions of polymers in dimethylformamide. "Modacrylic fibres" means fibres having a content of acrylonitrile (ACN) from 50 to 85% by weight.
(2) The Prior Art
In view of the pressing need, which has become felt in the latest years, of reducing the considerable inflammability of normal acrylic fibres, the manufacturers of these latter have endeavoured to obtain types of fibres having a reduced inflammability by using the same apparatus and the same process already used for the normal types, and have found an adequate solution of the problem in the use of certain monomers already employed for some time to produce the classic modacrylic fibres. The most suitable and easily available of all such monomers has been found to be vinylidene chloride.
Since a considerable amount of said comonomer is required in the copolymer to impart to the fibre a sufficient degree of resistance to the flame, its use produces an opacity of the product, especially when made by a wet spinning process wherein mixtures of water and dimethylformamide are employed as the coagulating bath. Consequently, the fibre does not possess the gloss and the dye yield required by the trade.
It is also known that the affinity for basic dyes of polyacrylonitrile based fibres may be substantially enhanced through the introduction of comonomers containing sulphonic groups, which further, in the case of wet spinning, improve the possibility of obtaining a glossy fibre. However it is impossible to obtain modacrylic fibres that are glossy when formed and maintain their glossiness after wet heat treatments, by a ternary copolymerization, viz. a copolymerization in which the sulphonic groups containing monomer and the vinylidene chloride are used concurrently with one another and with acrylonitrile, especially if enough vinylidene chloride is used to impart to the fibre produced, an antiflame quality corresponding to a LOI index not lower than 26% and if the coagulating bath consists of mixtures of water and DMF (dimethylformamide). The symbol "LOI", viz. Limiting Oxygen Index, indicates the minimum oxygen content in the air which is required to cause combustion of the product under the test conditions set forth in ASTM-D 2863-70. Products having a LOI value greater than or equal to 26 are to be considered as having a low inflammability.
There was therefore a true commercial need for low inflammability, modacrylic type fibres, concurrently maintaining:
(a) a good glossiness and consequently a good dye yield even after hydrothermal treatments;
(b) physico-textile characteristics as close as possible to those of the normal acrylic fibre, and for a process for their manufacture.